1. Technical Field
The invention relates to loud speaker horns and more particularly a horn accommodating planar type electro-acoustical devices or linear arrays of transducers.
2. Description of the Problem
Large space, public sound systems rely on a combination of loudspeaker types to achieve efficiency, wavefront coherence, a broad and level audio frequency bandwidth and good coverage of an audience located in the space. A foundational element in many such public sound systems is a line array of multi-transducer loudspeakers. A line array, in its classical form, consists of a vertical row of “closely spaced”, cone type, direct radiator acoustical drivers set in a baffle. In this arrangement adjacent acoustical drivers are mutually coupled to reduce the spread of the sound in the plane comprising the axis of alignment of the drivers and to promote even diffusion of the sound energy in an expanding half cylinder having the axis of alignment as its center. Since line arrays are typically oriented with the axis of alignment vertical. Hence, in this patent the term “vertical” should be taken as corresponding to the axis of alignment of the speakers, or center axis of a planar type device.
Mutual coupling of the acoustic drivers results from the acoustic drivers being identical, producing the same sounds and being closely spaced. What constitutes “closely spaced” is a function of the highest audio frequency that the array is intended to produce, but roughly means that the center of each speaker cone should be spaced from adjacent cones by no more than a quarter wavelength of the highest frequency sound the array is intended to reproduce. Audible sound ranges in wavelengths from about 17 meters at 20 hertz to 1.7 cm at 20 Kilohertz. The smallest direct radiator speakers used are usually on the order 10 cm. allowing sound reproduction up to about or a little greater than 3 Kilohertz. This provides for good speech intelligibility but is less suitable for amplification of music into large spaces.
Another type of electro-acoustic transducer is the planar, or ribbon, type device, which operates like a line array with no spacing between transducer centers. These devices are thus capable of reproducing the highest human discernable frequencies while keeping the sound field compression characteristics of a linear/line array. Ribbon type audio transducers date back to the early twentieth century. An example of an early ribbon type audio transducer is disclosed in U.S. Pat. No. 1,809,754 for an “Electrostatic Reproducer” to Steedle. Ribbon devices resemble an elongated flat panel and produce sound from a vibrating flat surface. In effect a ribbon or planar is a line array of infinitesimal elements positioned directly adjacent one another, i.e. a line array having zero spacing between mutually coupled drivers. This in turn means that a planar has no practical upper frequency limit in the human audio range. Unfortunately, as observed by Adamson in U.S. Pat. No. 6,343,133, ribbon tweeters have had limits in sensitivity and power handling capacity preventing application of the devices in replacing high frequency compression drivers in systems for large spaces. Planars can also suffer from selective frequency cancellation due to out of phase reflection issues from the mounting enclosure used in linear array units.
Due in part to the perceived problems with planars, several attempts have been made to produce a device that behaves like a planer but is constructed using horn loaded, conventional mid or high frequency drivers. Such a device is usually intended to be used to reproduce sound over a broad frequency range. Precursors to and examples of these devices are represented by U.S. Pat. Nos. 4,344,504 to Howze, 5,163,167 to Heil, 6,343,133 to Adamson, and 6,394,223 to Lehman. The proposed systems have obtained some of the performance, high frequency fidelity and efficiency gains of a planar and some have incorporated rectangular waveguide.
Another approach to obtaining a sound reproduction system having a broad bandwidth is that of U.S. Pat. No. 6,411,718 for Sound Reproduction Employing Unity Summation Aperture Loudspeakers. Here a pyramid-shaped loudspeaker (essentially a rectangular section horn where all four sides have a straight conic profile) is used to allow inclusion of a plurality of loudspeaker types to produce a coherent point source. The highest frequency loudspeaker is placed at the pyramid's apex and lower frequency loudspeakers are introduced progressively along the pyramid toward the mouth of the horn, which corresponds positionally to the pyramid's base. The '718 patent discusses reasons for using a straight flare rather than a more conventional exponential or hyperbolic flares, explaining that the “slowing” flare rate is “suitable for a lower frequency” where the throat of a loudspeaker is located relatively close to the mouth.